This invention relates to minimizing noise sources generated by the rotation of a propeller in air.
There are a variety of aircraft propellers, each designed for particular applications and cruise speeds. Some aircraft can vary thrust from the propeller by changing the revolutions per minute (RPM) while maintaining constant collective pitch, whereas other aircraft generate different levels of thrust by keeping the propeller RPM constant and employing variable pitch blades. Of course, different propeller designs have a different number of blades.
FIG. 1 illustrates an exemplary propeller 105 having 6 blades 110 supported by a hub 115. One such propeller having 6 blades is used by the C-130J Hercules aircraft and has a diameter of 14 feet. The propeller of this aircraft is operated at a constant RPM with thrust controlled by varying the pitch of the blades.
Regardless of the types of propeller thrust mechanisms, rotation of an aircraft propeller generates noise due to displacement of air caused by the impingement of a blunt body (thickness noise) and pressure fluctuations due to lift and drag (loading noise). There are some tertiary effects; however thickness and loading noise are of primary concern.
FIG. 2 illustrates the directivity field of the two types of noise sources generated by propellers. An aircraft fuselage 205 includes a motor 210 connected by a shaft 215 to hub 220. A plurality of blades 225 are mounted to and rotate around the hub 220. Thickness noise 230 is caused by repeated fluid/air displacement by one or more rotating elements, i.e. blades 225 that are relatively blunt. The thickness noise 230 is generally centered in the plane of rotation and propagates radially outward away from the tips of the blades. Loading noise 235 is caused by the alternating lift/drag dipole with lift occurring at a given position with one of the blades being closest to that position and drag occurring at the given position when two adjacent blades are equidistant from the position. The loading noise 235 generally propagates perpendicularly away from the plane defined by the propeller blades.
Propeller generated noise is undesired. It is a nuisance for crew and passengers on many propeller driven commercial aircraft. Propeller noise is of equal to or greater importance in the military theater; however, ramifications of such noise has potentially serious consequences when military aircraft must operate in contested airspace where an enemy may discern the presence and/or location of such an aircraft due to perceived propeller noise. Because of this, penalties to efficiency and/or aerodynamic performance may be accepted in favor of a less detectable propulsion mechanism. Thus, minimizing propeller generated noise would be welcomed in both commercial and military communities.